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Figure One: A flowing mountaian stream

The purpose of this page is to focus on using low technology hand methods to find the flow of streams and small rivers. Flow can also be found for pipes, sewage systems, and household appliances. People use flow data for Microhydro systems, waste-water information, settling rates, water table statistics, etc. To find the flow of larger water bodies such as dams or major rivers, high technology is used such as meters. Meters are described briefly in this page.

Contents

Flow

Flow is the total volume of a fluid that flows past a fixed point in a river or stream over time. It is comparable to the speed at which a volume of fluid travels as seen in Figure One. Volumetric flow rates can be measured in various units such as:

Measuring Flow

Bucket method

The Bucket method is a simple way to measure the flow rate using household items. It requires a stopwatch, a large bucket, and two to three people.

Measure the volume of the bucket or container.

Find a location along the stream that has a waterfall. If none can be found, a waterfall can be constructed using a weir (see Figure Four).

With a stopwatch, time how long it takes the waterfall to fill the bucket with water. Start the stopwatch simultaneously with the start of the bucket being filled and stop the stopwatch when the bucket fills. The bucket should not be filled by holding it below the surface of the stream because it is not the true flow rate.

Record the time it takes to fill the bucket.

Repeat steps two and three about six or seven times and take the average. It is a good idea to do a few trial runs before recording any data so that one can get a feel for the timing and measurements required.

Only eliminate data if major problems arise such as debris from the stream interfering with the flow.

The flow rate is the volume of the bucket divided by the average time it took to fill the bucket. [2]

Using this data, the volumetric flow rate (Q) is equal to the volume of the bucket (v) divided by the average time (t).
where
so t=13.5 seconds
and v= 5 gallons

The flow rate Q= 0.37 gallons per second
or 22.2 gallons per minute

Float method

Figure Three: Finding the flow rate using a float and a meter stick.

The float method (also known as the cross-sectional method) is used to measure the flow rate for larger streams and rivers. It is found by multiplying a cross sectional area of the stream by the velocity of the water.

Locate a spot in the stream that will act as the cross section of the stream.

Using a meter stick, or some other means of measurement, measure the depth of the stream at equal intervals along the width of the stream (see Figure Three). This method is similar to hand calculating a Riemann sumfor the width of the river.

Once this data is gathered, multiply each depth by the interval it was taken in and add all the amounts together. This calculation is the area of a cross section of the stream.

Decide on a length of the stream, typically longer than the width of the river, to send a floating object down (oranges work great).

Using a stopwatch, measure the time it takes the float to travel down the length of stream from step 4.

Repeat step five 5-6 times and determine the average time taken for the float to travel the stream.

Divide the stream length found in step 4 by the average time in step 6 to determine the average velocity of the stream.

The velocity found in step 7 must be multiplied by a friction correction factor. Since the top of a stream flows faster than the bottom due to friction against the stream bed, the friction correction factor evens out the flow. For rough or rocky bottoms, multiply the velocity by 0.8. For smooth, muddy, sandy, or smooth bedrock conditions, multiply the velocity by a correction factor of 0.9.

The corrected velocity multiplied by the cross sectional area yields the flow rate in volume/time. (Be sure to keep consistent units of length/distance when measuring the cross section and the velocity eg. meters, feet)[3]

Weirs

Figure Four: An example of a V-notch weir

Weirs are small dams that can be used in measuring flow rate for small to medium sized streams (a few meters or wider). They allow overflow of the stream to pour over the top of the weir, creating a waterfall, as seen in Figure Four. Weirs increase the change in elevation making the streamflow more consistent which makes flow rate measurements more precise. However, it is very important that all the water in the stream be directed into the weir for it to accurately represent the stream flow. It is also important to keep sediment from building up behind the weir. Sharp crested weirs work best. [4]
There are many different types of weirs which include broad crested weirs, sharp crested weirs, combination weirs, V-notch weirs and minimum energy loss weirs. [5]

Meters

Meters are devices that measure the stream flow by directly measuring the current. There are many different types of meters by the most common is the Pygmy meter, the vortex meter, the flow probe, and the current meter: They are briefly described.

Pygmy meter: a wheel is rotated by water flow and the rate of the rotation signifies the water velocity. It is primarily used in measuring discharge. [6]

Vortex meter: velocity is proportional to the downstream frequency of the vortex flow and is read on a digital readout. It is used for measuring flow in pipes. [7]

Flow probe: the flow turns a propeller that sends the water velocity data to a digital readout display in ft/s or m/s [8]

Current meter: electronic pulses determine water velocity. Can be used in large bodies of water like oceans to measure the current. [9]